Well, Paizo has promised that the final version of Pathfinder will fit that bill.
So here's a suggestion for you. In my opinion, the reason 3.5 gets un-fun at higher levels is because of three main problems:
1. BAB gets too high for certain classes after a while
2. The disparity of saves makes it so that you're always sure to make certain saves and always sure to fail other saves.
3. Too many buffs, which are allowed to keep stacking endlessly, which complicate things to the point where each action in combat becomes a major feat of accounting.
My solution is to stick with 3.5, but make some modifications, as follows:
1. Use the 4th edition system. Rather than every class having its own BAB progression, it is instead 1/2 level + ability modifier for all classes.
2. Again, use the 4th edition system. All saves for all classes are 1/2 level + ability modifier.
3. Limit all characters to a maximum of 5 buffs at any given time. If that's still too much, reduce to 3.
These would be simple changes to implement, and I think they would be hardly noticeable at lower levels, but they would make a huge difference at higher levels.